In almost all solidification processes, the crystallization of a bulk liquid volume occurs through favorable interaction between the melt and internal or residual catalysts which act as heterogeneous nuclei. The heterogeneous sites can be melt impurties or Other external catalysts inherent to most casting processes. Although most naturally occurring or intrinsic catalysts may be activated at undercoolings of only a few degrees, coarse constituent product phases and/or coarse columnar grain structures will usually develop in the absence of inoculant particle additions that provide for the individual or combined effects of a high nucleation frequency and a constitutional growth deterrent during solidification. Therefore, in order to modify or refine as-cast solidification microstructures, it is common practice to intentionally inoculate a melt with particle catalysts that surpass the catalytic activity of other less potent heterogeneous nuclei in contact with the bulk liquid. By using these inoculation procedures, the added catalysts can promote the formation of refined, equiaxed as-cast grain structures, refined primary phase constituents, or morphologically altered phases, all of which can either enhance desired mechanical properties or improve the surface finish of metal and alloy castings.
Given this information it is noted that inoculation treatments fall within two classes. In the first class, inoculation treatments are used to promote the nucleation of modified product phase morphologies. For example, copper phosphide particles are added to aluminum-silicon alloys to produce a refinement of the silicon phase size while the addition of sodium to an aluminum-silicon alloy will modify the growth morphology of the primary silicon. In cast irons, ferrosilicon additions promote the nucleation of graphite while additions of magnesium result in the formation of spheroidal or nodular graphite in ductile cast iron. The second class of inoculation treatments are used to alter the relative abundance and distribution of intentionally nucleated phases and usually involve the addition of particle agents that promote the formation of fine, equiaxed as-cast grain structures. Examples include the refinement of magnesium alloys via zirconium or carbon additions, the grain refinement of copper by iron, cobalt, or zirconium, and the addition of arsenic or tellurium to lead alloys. In addition, there also exists a vast commercial and experimental database concerning the grain refinement of aluminum and aluminum alloys through master alloy additions containing inoculant particles inherent to the aluminum-titanium and aluminum-titanium-boron systems. Moreover, the aluminum-titanium and aluminum-titanium-boron systems are generally used as a paragon for theoretical interpretation of inoculation and corresponding physical mechanisms concerning the realm of heterogeneous nucleation catalysis.
Although mechanisms that influence effective heterogeneous nucleation catalysis are not fully understood, based upon the prior art and associated theoretical interpretations, inoculant particles must provide two fundamental characteristics that influence the effective grain refinement of castings. These characteristics are a kinetic restriction that optimizes a high nucleation frequency and a chemical solute restriction that provides for a constitutional growth deterrent of the nucleated crystals. See. e.g., D. Turnbull and B. Vonnegut, Ind. Eng. Chem., 44, 6, 1292 (1952); I. Maxwell and A. Hellawell, Acta Met., 23, 229 (1975). Both characteristics are necessary to insure a high density of fine grains or crystals. The former condition is maximized for particle catalysts that provide surfaces possessing similar crystal chemical properties and low crystallographic misfit orientations for nucleation of the crystalline solid, while the latter condition is optimized when a peritectic type reaction occurs between the inoculant particles and the liquid and solid phases. As an ancillary condition, the nucleation process must be uniformly activated by the inoculant particles at essentially negligible undercooling levels in order to circumvent anomalous nucleation and growth of coarse columnar structures occurring in regions of castings void of effective catalyst particles.
Although the use of inoculation treatments is common in commercial practice, a considerable lack of reproducible effectiveness exists with the use and performance of most grain refining agents. For example, under ideal conditions the number of grains in a refined casting should approach the total number of inoculant particles added to the melt prior to solidification. However, bulk grain density measurements of refined aluminum castings reveal that only 1-2% of the total number of added inoculant particles actively promote the grain formation. See M. M. Guzowski, G. K. Sigworth, and D. A. Sentner, Met. Trans., 18A, 603 (1987). Further, it is virtually impossible to visually identify the active nucleant particles present within a bulk casting that contains a multitude of inactive or less active particles of variable sizes, shapes, morphologies, chemical identities, and crystal structures. For the case of aluminum, for example, visual identification of the potent inoculant population in bulk refined ingots is not possible because solidification morphologies associated with the nucleation and growth of aluminum on effective particle catalysts are usually consumed during matrix coarsening in slowly cooled ingots. Therefore, given a maximum 2% effectiveness in commercial inoculation efficiency, examination of typical inoculant particles within a bulk refined ingot will not yield an accurate representation of the actual nucleation parameters responsible for effective catalysis of grains by a minority of the inoculant particle population. Given the low efficiency and lack of certain understanding surrounding heterogeneous nucleation catalysts and inoculant operation, there is a need for a technique that allows for the identification of effective catalytic agents.